The present disclosure relates generally to wireless communications. More particularly, the present disclosure relates to ATSC transmitter identifier signaling.
Positioning receivers that are based on time-of-flight, such as GPS receivers, rely on extremely precise measurements of signal arrival times from multiple transmitter sites. Each relative time-of-flight measurement, when combined with the propagation speed of the signal and precise knowledge of transmitter positions, represents a constraint on the possible receiver location. An estimate of position can be formed by combining several such constraints.
This approach to positioning has been considered either unfeasible or, at least, extremely challenging for broadcast transmissions that use single-frequency network (SFN) technology, such as typical with Digital Video Broadcasting (DVB), Integrated Services Digital Broadcasting-Terrestrial (ISDB-T), Digital Audio Broadcasting (DAB), and Advanced Television Systems Committee Mobile/Handheld (ATSC-M/H) networks. In an SFN network, geographically dispersed transmitters emit time-synchronized replica signals. Hence, the signals arriving from different towers are not distinguishable, and it is not straightforward to associate the arrival of particular quanta of signal energy with any particular transmitter site.
The situation is made even more difficult by multipath, in which a signal may reflect and refract as it transits to the receiver along many different paths, each of which may overlap and either cancel or obscure the arrivals of weaker signals from other transmitters.
Another obstacle to positioning for all SFNs is high near-far ratios. That is, the ratio of received power from different transmitters may be extreme. Since all transmitters share the same frequency in an SFN, a high near-far ratio makes it difficult for receivers to reject a strong signal in favor of a weaker one. A consequence of the near-far effect is that the weaker signals may not be detected and hence not used for ranging, or may suffer increased ranging errors. In the limit, this effect can prevent positioning altogether, as a single very “loud” signal can drown out all others. The better a receiver system is at rejecting near-far effects, the larger the potential coverage area of the positioning system.
As the nomenclature suggests, near-far effects frequently occur due to the path loss difference between a distant and nearby transmitter. Large near-far ratio can also be the result of anisotropic building attenuation, fading, or differences in transmitter effective radiated power (ERP). Even GPS, despite near-uniform outdoor power flux, can suffer from high near-far ratio due to the differential attenuation of signals from different satellites when indoors.
Some SFN standards have defined “watermark” overlay signals intended for ranging and/or channel characterization. These overlay signals are transmitted in synchrony with the main signal, but at far lower power levels. For example, the ATSC A/110 standard defines a 64K-chip 2-VSB Kasami sequence that can be “buried” between 21 and 39 dB below the main 8-VSB signal. To a receiver attempting to demodulate the main signal, such a buried signal has an effect similar to Gaussian noise and, if buried sufficiently, will have no significant effect on the reception characteristics of the main signal. A receiver that is ranging from the watermark correlates against the Kasami reference sequence, taking advantage of the consequent processing gain to reduce the interference caused by the main 8-VSB signal.
Though watermark-style signals can be used for positioning, they are not effective in environments with even moderate near-far ratio. For example, consider an A/110-compliant SFN signal in which the watermark has been buried by 39 dB. One cycle of the Kasami code has a processing gain of 10*log(216)=48 dB. Assuming that 13 dB SNR is the minimum required for accurate peak classification and ranging, and assuming 17 dB of integration (˜0.3 s) is employed to reduce the interference created by the stronger 8-VSB signal, a usable dynamic range of only 48−39−13+17=13 dB remains. That is, if the stronger signal is just 13 dB more powerful than the weaker one as measured at the receiver, ranging won't be possible from the weaker signal. In real-life scenarios with terrestrial transmitters, near-far ratios can exceed that value by a factor of 1000 or more.